The constant photocurrent method (CPM) is widely used as a means of determining subband-gap absorption spectra in thin-film semiconductors, particularly, hydrogenated amorphous silicon materials.

It has been observed experimentally that photoconductivity of a semiconductors is often connected with the generation rate G by a power law:

The power law exponent (Rose factor) characterizes the recombination properties of the material. Under uniform illumination ( alpha*d<<1, where alpha is the absorption coefficient and d is the film thickness) the generation rate G can be approximated by G=alpha* N, where N corresponds to the total incident photon flux. Thus, one can write:

In the case of materials with the continuous distribution of the density of states the power law exponent gamma can vary with the generation rate, and the proportionality Iph is proportional to G^(gamma) with = const. can be defined in a small range of G only. This fact was the main motivation to introduce the Constant Photocurrent Method CPM for a-Si:H.

At a first approximation the secondary photocurrent is proportional to

where e is the elementary charge, N is the number of incident photons over active sample area, N (1-R) [1-exp(- alpha*d)] is the number of photons absorbed in the sample with reflection coefficient R, absorption coefficient alpha and sample thickness d (no interference of light is taken into account), is the quantum efficiency, mu(0) is the electron drift mobility, tau(0) is the lifetime and F is the applied electric field strength.

In the subgap absorption region where the condition alpha*d<<1 is always fulfilled for a thin film, this equation could be simplified to

If we want to find the spectral dependence of the absorption coefficient alpha(E) from measured Iph(E), the fact that also other material related parameters may be in principle spectrally dependent could complicate the procedure. Fortunately, when additional independent information about the material is available and experimental conditions are carefully chosen, correct information about the absorption spectrum can be obtained.

In order to have photocarrier lifetime constant we need to stabilize the demarcation levels. When we suppose one dominating recombination center and one majority type of carriers, stabilization of the demarcation levels is possible by ensuring that the photocurrent passing through the sample is kept constant in whole CPM spectrum. Then taking into consideration all mentioned and supposing negligible dependence of reflection (index of refraction) we can write a simple proportionality using final equation

where N(E) is the number of photons necessary to keep Iph=const. This is the basic CPM idea.